Raster scan photoplotters of both planar and internal drum design are used in the fabrication of printed circuit boards. Planar photoplotters or imagers such as are disclosed and claimed in U.S. Pat. No. 4,851,656 and incorporated herein by reference have a planar surface for receiving a substrate. An optical exposure head is located on a movable gantry apparatus and is rastered above the substrate during exposure. Internal drum photoplotters have a cylindrical surface portion to receive the substrate. The exposure beam emanates from an optical exposure head and is scanned across the substrate. The optical exposure head is indexed along the longitudinal axis of the cylinder to complete the substrate exposure.
Similarly, optical scanners receive light from a surface which contains an image that is to be converted to an electronic equivalent. In most scanners, a document and/or artwork is placed on a surface and illuminated. Light containing the image is gathered by an electronic photosensor that is typically rastered about the surface. Both the photoplotter and the scanner are examples of scanning systems generally.
Internal drum raster photoplotters have inherent advantages over planar type scanners for several reasons, including simplicity of design and lower costs. However, both are subject to component tolerances which result in lower accuracy than would otherwise be possible. The drum surface is fabricated with inherent deviations from perfect cylindricity. Planar photoplotters or flatbed scanners are similarly afflicted by surface irregularities. These deviations result, in part, in scan lines on the substrate of varying length.
A compensation technique adaptable for use with a planar photoplotter or scanner is found in U.S. Pat. No. 3,555,254, incorporated herein by reference. Disclosed therein is a system for positioning a driven part in a numerically controlled positioning device. In the setting up of the system, the driven part is commanded to move to various positions spread over its field of movement and after it reaches each such position, its actual position is accurately measured to determine the error between the commanded position and the actual position. The values of the errors thus determined are stored in computer memory as a table of error values versus part position. Thereafter, as the driven part is moved to different positions relative to the reference member, the computer memory is interrogated and error values from the table are used to correct the commands transmitted to the motor drivers to take into account the repeatable error associated with the position of the driven part. The '254 system further includes an input device for providing position input commands, one or more motors for driving the driven part, and a computer for converting the input commands into corrected output commands.
In the prior art, internal drum raster photoplotters have been built with components defect compensation. For example, MDA of Vancouver, BC markets an internal laser raster drum photoplotter with compensation. First, the deviations from true cylindricity are determined as a function of position on the cylinder portion surface. The commanded raster pattern is thereafter shifted by incremental pixels at the photoplotting resolution (i.e. 0.25 mil). Pixels are dropped from the image and other pixels are duplicated as needed. These commanded shifts have the unfortunate effect, however, of distorting the image and introducing abrupt shifts of 0.25 mil in the photoplot. For example, if a shift were to occur exactly on a circuit trace, it would distort the width of this trace by the 0.25 mil error, which can be detrimental to the performance of the circuit.
Another example of a known scanning system having compensation is disclosed and claimed in U.S. Pat. No. 5,291,392 which patent is incorporated herein by reference. The '392 method is characterized by method of providing compensation for inaccuracies in a scanning optical system that has a platen for receiving a substrate and a scanning means responsive to a clock signal for advancing relative to said substrate an optical beam across the substrate surface forming a series of pixels that constitute a scan line.
The '392 method includes the steps of generating command signal values for registration marks positioned about a substrate surface, exposing a calibration substrate to an optical beam so as to image said registration marks therein and generating signals indicative of the measured position of said registration marks in said substrate surface. The method also includes the steps of comparing the measured registration mark position signal values with the command position signal values to generate error signals corresponding to deviations of the measured registration mark positions from the command signal value positions and generating control signals to adjust the phase of a clock signal in dependence on the error signal magnitude, thereby removing the deviations from a scan line segment of the scan line.
The '392 system is limited in the sense that the preferred embodiment disclosed therein incorporates a tapped delay line for generating a plurality of clock signals each at a different phase relative to one another. There is no variability in the phase separation since the delay and number of taps are fixed. Compensation based on a delay line is, therefore, limited to systems with low spinner speeds. It would be advantageous to have a system for providing compensation for nonlinearities in scanning systems which avoid abrupt shifts in the written substrate and which are not limited to a single speed or, in systems with curved platens, a single diameter. The present invention is drawn toward such a system.